Estimation of the distance to a tag in a Radio Frequency Identification (RFID) system is a useful feature. It would provide for an improved method of cross-lane discrimination in RFID tolling stations, while also providing a platform to estimate vehicle speeds in the same type of application. While there are many techniques developed in prior art for range estimation, this approach uses digital components in the receiver. Patents in the general field include U.S. Pat. Nos. 6,600,443; 6,476,756 and 7,030,361.
Past methods include angle of arrival at multiple receivers, received signal strength indicator (RSSI), and variation of phase in a received signal. Each of these has their strengths but the present method particularly applies to a system with discrete (i.e. digitally sampled) signal inputs.